1. Field of the Invention
The present invention relates generally to dental alloys and more particularly to a dental alloy suitable for use with both high and low thermal expansion porcelains.
2. Description of the Prior Art
Metallic alloys are commonly fused to dental porcelains for the fabrication of crowns, bridges and other prosthetic appliances. The various properties of these dental casting alloys are defined by the composition of the alloys. Such an alloy should exhibit an adequate balance of the following properties:
1. A coefficient of thermal expansion which makes the alloy suitable for use with commercially available porcelains.
2. Biocompatability with conditions in the mouth.
3. Good bonding characteristics with porcelains.
4. Ability to provide an aesthetically pleasing final product.
5. Characteristics enabling it to process well during casting.
6. Adequate strength and ductility.
7. Low cost.
For many years an adequate balance of these properties was obtained by using precious metals such as gold and platinum as primary elements in dental casting alloys. However, as the cost of precious metals increased, alternative alloy compositions were sought. As a result, cobalt and nickel based alloys became of great interest, but many of these alloys did not provide a satisfactory balance of properties with respect to the high degree of precision required during the casting step for some applications. Gold/palladium also became popular alternatives for use in dental castings. However, substantial and costly amounts of gold were still required to provide an adequate balance of properties.
With ever-increasing prices of gold, interest has turned to palladium-silver alloys. These alloys provide a reasonable balance of properties at a much lower cost than gold alloys. Yet, the use of palladium-silver alloys has not proven entirely satisfactory. For example, silver is known to cause greening of dental porcelains during the fusion process. One way of reducing the greening of the porcelains is to reduce the alloy's silver content and increase the alloy's palladium content, however, this method of reducing porcelain greening has been limited by the amount of palladium which can be used. For example, U.S. Pat. No. 4,350,526 states that a palladium content greater than 60% can adversely affect the overall balance of properties of the resulting palladium-silver alloy. U.S. Pat. Nos. 4,194,907 and 4,350,526 discuss the use of silicon in a palladium-silver alloy in order to prevent the silver from discoloring the porcelain, however, U.S. Pat. No. 4,261,744 indicates that more than 0.25% silicon in a high palladium content alloy can produce a cast product with crack and void defects.
Silver free palladium alloys have been proposed to avoid porcelain discoloration problems. Yet, these alloys have not been found entirely satisfactory either. For example, palladium is known to pick up gases during melting. This results in a porous casting which makes adhesion to dental porcelain difficult. U.S. Pat. No. 4,261,744 teaches that silicon can be used to prevent gas absorption in molten palladium, but, as previously mentioned, no more than 0.25% silicon can be incorporated in the alloy without producting castings with crack and void defects. In addition, palladium alloys tend to be compatible with only low thermal expansion porcelains; their compositions render then incompatible with high thermal expansion porcelains such as CRYSTAR.TM..